Joined: 26 Jan 2003
|Posted: Tue Mar 06, 2012 4:59 am Post subject: Summary of E-Conf -Days 7 & 8
|TRAINING - - - -- important for EVERY ONE handling CONCRETE
Just some humor for a while in this connection
Santa Singh could not appreciate curing of concrete while constructing his village house rcc
roof slab and asked his brother Banta Singh that why we are watering the 'Lanter' (rcc slab),
Banta replied if 'Roddi (concrete ) is allowed to drink water now, it will become strong and
if not , it will not only drink us but eat us too(i.e. fall on us) in future.Probably Banta Singh
was right even as a layman handling concrete.
Training must be given to every one handling the concrete , whether engineer or a Labor
It always helps a along way .
We have , by now, completed eight days of the e-conference on Concrete Durability and time
Postings on days 7 & 8 - - - - - - -
We have got very enlightening , educative and exceptional posting from Dr Narayan Nayak
MD ,Gammon Reality , who first thanked the Moderators of e- conf Dr N Subramanian and
Mr Vijay Kulkarni along with Madam Alpa Sheth for being brain behind the conference.The contents
of this posting are par -excellence..He has discussed about various issues for durability alongwith
discussion on concreting methodology for columns, piles cut-off etc .I am finding it difficult to brief the
contents since each issue in his posting is beyond skipping .
Blended Cements - - concrete blended with Fly Ash, GGBS ,Metakaolin , Mico silica
help in Durability and sustainability.The availability of FA is to the extent of 180 million Tons/annum
in our Country. BIS restricts replacement of cementitious materials by Fly ash upto 35% only but he
feels that it may be increased to 50% since BS provisions allow 55% as upper limit. Production of
Pond Ash is about 125 milllion tons/annum and only 15% is utilized for Bricks& embankments.
Disposal is a problem and it may be used as Fine aggregate for increased Strength and durability.
Copper slag is produced significantly in Gujarat ,Jharkhand and TN and it can be used for Fine
Aggregate as it meets gradation requirements.In absence of Codal provisions , there is resistance
for use of Cu Slag.
Curing is also emphasized and it is not satisfactory .Research on this crucial activity is further needed
on Internal self curing compounds. Present structures are heavily reinforced and
use of Self compacting Concrete must be encouraged to avoid honeycombs. Column starters
concreting is of concern since concrete is rarely vibrated/compacted . Gammon India avoids
column starter by using appropriate Form-work methodology. Joints are potential source for corrosion
of reinf.and hence in Gammons seamless joints are achieved thru suitable arrangement in form work.
Binding wires with length more than required will encroach cover zone and trigger corrosion. Concrete
above pile cut-off level is mostly done using percussion method or by crushing thru hydraulic m/c ,but
both methods are not desirable since lead to cracks in concrete below . Gammons recommend
removal of such concrete at its green stage itself and compacting the concrete below cut off level.
L-bends at bottom of reinf cage for Piles must be avoided for concreting and proper covering reinf
at bottom. For Under-ream piles , concreting at slow pace needs to be done for whole concreting in the Bulb formation. Mr Narayan Nayak also cited the concreting of 3.5m thick raft with M50 concrete in
single pour without bothering for high temps (using OPC -53G @ 460kg/cum and micro- silica @ 100Kg/cum) Temp of 86 deg recorded.He felt that this could be achieved better by using FA concrete without use of silica with cementitious content of 430Kg/cum (FA35%).Cementitious content can be further reduced using 40mm down aggregate and conc being laid in layers. Even M-60 grade conc can be used without micro silica. He also stressed the importance of TRAINING at site.
I must not refrain to express my deep appreciation and thanks to Dr Nayak for such wonderful
substance of the posting which may benefit one and all for producing durable and sustainable
Er Vijay Shah wants to know de-shuttering time for concrete work with RMC .Contractors have
tendency to remove shuttering early .Also RMC when reaches site is in plastic stage with plastic
shrinkage cracks.Discussions may also address problems of drying shrinkage cracks and plastic
shrinkage cracks and when & how to repair them. Responding to this, Er B S Nagesh stated
that they are working in the direction of providing instrumentation to the builders/contractors and
these sensors/instruments may provide the strength level of poured concrete . He also referred to the
article of NBM & CW.
Dr N Subramanian posted very interesting article in econference portal on 'Iran's Bomb proof-concrete'.
Er Jignesh V Chokshi thanked Dr Nayak for raising many other points ,not raised so far in the
e conf. .The pile cut off methodology of cutting it during green stage is definitely good. However in
cases where cut off level is deep enough or variation in levels , how the methodology can be
applied since piling rig cannot be removed before piling is completed. Dr Nayak responded
to this explaining the ways/tools used for cutoff of conc while at its green stage even for deep levels
He also gave reference to his Book 'Foundation Design Manual" regarding these tools and
methodology. For very deep cut -off levels keeping temporary liners in position upto completion of
excavation may be solution when pile group is small. In large pile group , liners are not advisable
to be kept upto excavation and Bore after removal of green conc , can be filled with sand and
followed by liner removal upto cut off level . He has also mentioned about EAR KEY arrangement
between Liner and separate liner above cut-off.
Dr Nayak also responded to the query from Er Moruf and reaffirmed the use of FA,GGBS, Metakaoline,
Micro -silica for concrete durability.Reduction in water binder ratio is further good. He feels that FA
content upto 50% for foundation/sub-structures and 30-35% for superstructures is ideal. FA conc gains
strength slow but addition of Alcofine or Chemical admixture may overcome this problem. SCC
is used in heavily reinforced structures to avoid honey combing and cost wise , may not differ from
other concrete since vibration cost is eliminated.
Er Murtaza Kagdi lists various key points for avoiding shrinkage cracks like use of optimum water,
preventing rapid loss of moisture when conc is plastic stage by spray aids or by using plastic
sheets for plastic shrinkage cracks .Concrete must be covered immediately after pouring by
poly thene sheets.
Er Alok Bhowmick thanking Mr Nayak seeks advice on the FA content of 50% to be adopted at site
or at RMC set up or at both irrespective , whether our standards in present will allow increase
in FA content and any additional precautions to be taken at site for this.
Our respected sefi member Er HS Harsoda faced problems of login to econf and informed on other
portal that use of Quarry Rock dust improves concrete Durability and this will be economical too
since natural sources for sands are depleting.He also highlighted various durability studies with
basalt rock dust and marble sludge powder.
Er Jignesh referred to advice of using FA concrete with FA content 35% and 55% by various
country codes.Fly ash is a bye product in coal based plants. But with No coal based plants
scenario in future with hydro, nuclear, gas and solar power plants , Use of FA is debated
and also whether it is compulsion or alternative. Have Ancient structures use Fly-Ash ,if yes ,
then where was source availability.
Mr B S Nagesh Proprieter of SAMHITHA INNOVATIONS informed about an article published
in latest issue of NBM &CW of Mr John Gneadinger of Con-cure Corp (USA) mentioning the
importance of mix, proper curing , posible places of strength reductions in the open areas
of form work , how to have best mix design with temp mentioned etc. Appreciate the gesture.
Er Abhio wants to know that when IS 226, IS 8112, and IS 12269 specify only minimum fineness
of cement, it allows cement manufacturers to pack as same cement for 33, 43 & 53 Grades
and that isl not be good for projects requiring large conc pours for massive structures
needing slow set time and gradual HOH.
Er Satish Kulkarni replying to Er Vinod's posting that other cementitious materials are
not popular and not acceptable to workmen (masons ), states that as an engineer it is duty
to be well aware of newer materials and their suitability.If successful , everyone including
masons will appreciate.
with many thanks and regards to all and hoping for more learning on e- conference
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